Currency so you do OSRS Gold not need to spend a single penny but of course it's advised to at least pay for one or two months of membership to grow your account in order to have reliable options to make enough money to buy bonds.
Making money in free to play is hell. There's no quick and reliable way to make money and the methods that you may find googling, the spots are usually populated by bots. Bots aren't a problem in members world though so don't worry.
You train your combat stats as well whether that be melee, range, or mage. Certain tasks are better off keeping your distance, such as dragons so range and magic will be better off for those and of course, certain tasks are better off using melee.
It's a great way to make your character stronger AND make RS gold money.At higher levels, one slayer task can earn you over 1 million gold easily. Another fun aspect is a lot of monsters in slayer can drop something called clue scrolls which is basically a treasure hunt where you solve riddles.